Under "Sudoku Solving Guide", there is an example for "Locked Candidates (type 2)". Could someone please explain why box 5 can't have 7 in Column 4 or Column 6 (highlighted in red). With all the given digits shown, I somehow failed to understand this topic.

In the example column 5 has four cells filled in, and five empty cells (r2c5, r4c5, r5c5, r6c5, and r8c5). There's a "7" at r2c3, and another "7" at r8c9.

-- Clearly the "7" in column 5 cannot appear at r2c5, because of the "7" at r2c3.

-- Similarly, the "7" in column 5 cannot appear at r8c5, because of the "7" at r8c9.

-- Therefore the "7" in column 5 must appear at r4c5, r5c5, or r6c5. In other words, the "7" in column 5 has to lie in box 5.

-- But saying that the "7" in column 5 lies in box 5 means the same thing as saying that the "7" in box 5 lies in column 5. And since the "7" in box 5 has to be in column 5, it can't possibly be in column 4 or in column 6.